Mouse embryos can be made to develop in vitro continuously from the blastocyst stage (stage 6 of Witschi, 1972) to limb-bud stage (stage 18). The sera from different species of animals promote the growth and differentiation of mouse embryos only at certain stages. To explain the results it has been tentatively hypothesized that sera from different animal species contain different factors (EGDF) which promote the growth and differentiation of mouse embryos at different stages. Among five factors which have been proposed, we will study the EGDF-3 which promotes the growth and differentiation of embryo proper from stage 11 to 15. In a medium deficient in EGDF-3, mouse embryos developed empty sac which consisted of embryonic membranes without embryo proper (equivalent to blighted ova of human spontaneous abortuses). Human cord serum and human placental tissue will be used to characterize and fractionate the active components. The separation will be accomplished on the basis of molecular weight and isoelectric point. After purification the purified factors will be labeled. The labeled factors will be used to determine the possible target tissues. The long-term goal is to understand the factors which control the normal and abnormal mammalian development in early embryogenesis.